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The problem with presuppositionalism

There are basically two routes that apologists will take in trying to argue the case for the existence of God. They often subtly shift back and forth between the two – perhaps not even really being aware of what they're doing – but they can indeed be delineated by two broad categories. The first is that they'll attempt to argue that an examination of evidence leads us to the existence of God. This is essentially (though often unstated) conceding the importance of an empirical epistemology, and using empiricism to lead to God. This usually fails pretty hard, so they bring out what Stephen Law calls the "nuclear option": presuppositionalism.

That oversized mess of a word means just what it sounds like: the believer argues that one must presuppose the existence of God in order to have any kind of valid epistemology at all. This isn't some fringe idea at all – Frank Turek argued it recently in his debate with David Silverman (I suffered through some of it); it's the root of Alvin Plantinga's "evolutionary argument against naturalism"; and David Marshall, Tom Gilson and Peter Grice (among others) argued it in the apologetics essay collection "True Reason".

It seems like it should go without saying that these two routes are incompatible, though apologists rarely if ever get taken to task for the error. Think about it: you presuppose that God exists, and then the evidence you examine leads you to affirm God's existence. If that ain't circular reasoning, nothing is. If presuppositionalism is valid, then God's existence is self-evident. Apologists rarely if ever use this exact language (probably because it betrays how stupid presuppositionalism actually is), but it's the inevitable logical conclusion – God's existence can't be supported by evidence even in principle, because without God you couldn't have evidence in the first place.

A disturbing lack of alternatives
One of my all-time favorite posts from the mighty Sean Carroll is one called "Faith and Epistemological Quicksand". He didn't use the word, but it was actually an incisive rebuttal of presuppositionalism. He said (emphasis mine),

Any time we have beliefs of any sort, we need to admit the
possibility that they are incorrect. Even if we have think that some
result has been reached by nothing but the application of pristine
mathematical logic (e.g. the ABC conjecture),
it’s always possible that we simply made a mistake — have you ever
multiplied two numbers together and gotten the wrong answer? Certainly
in an empirical endeavor like science, we recognize that our theoretical
understanding is necessarily contingent, and are constantly trying to
do better, via more precise and far-reaching experimental tests. These
are methods of reaching knowledge that have built-in methods of
self-correction.

So what about faith? Even if your faith is extremely strong in some
particular proposition, e.g. that God loves you, it’s important to
recognize that there’s a chance you are mistaken. That should
be an important part of any respectable road to knowledge. So you are
faced with (at least) two alternative ideas: first, that God exists and
really does love you and has put that belief into your mind via the road
of faith, and second, that God doesn’t exist and that you have just
made a mistake.

The problem is that you haven’t given yourself any way to
legitimately decide between these two alternatives. Once you say that
you have faith, and that it comes directly from God, there is no
self-correction mechanism. You can justify essentially any belief at all
by claiming that God gave it to you directly, despite any logical or
evidence-based arguments to the contrary.

In a nutshell, that's the problem with presuppositionalism. If you presuppose that the existence of God is foundational to any valid epistemology, then you have no way of knowing whether you're right or wrong about your presupposition. All evidence doesn't merely lead to God's existence, but the mere conceptualization of evidence itself necessitates God's existence. That means there can be no self-correction mechanism – no methodology by which the assumption of God's existence could be falsified, since any rational examination of the evidence would include the a priori assumption of God's supposedly self-evident existence. Apologists deserve a gold medal for such sophistic gymnastics.

And when you think about it, isn't it pretty hilarious that one of the great triumphs of 'sophisticated' apologetics is that theologians have come up with an elaborate rationalization for assuming, a priori, precisely what they are trying to prove?